In KOH-etching of 100 silicon material, the crystallographic properties of the silicon wafer are exploited to etch precisely rectangular openings in the wafer surface. Vertical walls do not arise in the depth, but rather inclined flanks having an angle of 54.7°. This property is also a crystallographic property of the 100 wafer material. The inclined flanks correspond to the 111 planes and actually have an etching stop character in KOH. The 111 planes have the lowest etching rates in silicon crystal by far and may thus be used to provide well-defined openings having defined inclined walls.
In KOH etching starting from one side of the silicon wafer, walls having an angle of 54.7° to the 100 plane always result in the depth. Steeper flanks may thus not be implemented from a crystallographic viewpoint.
However, it is desirable in some micromechanical structures to have the steepest possible flanks to obtain the greatest possible mechanical stability, or, for example, to be able to place cap wafers having the largest possible bond area as close as possible to bond pads.